Raise Parameter Error if both encryption and stdin upload are specified

Small (and hopefully temporary until functionality can be implemented) change to notify user when they try to use "-" to upload from stdin and -e to encrypt before uploading that this will fail.  Otherwise, it appears to work but uploads an encrypted file with no contents and reports success. 

Fixes #351

Old output:
```
cat mytest | s3cmd put -e - s3://dump/test/mytest
WARNING: Module python-magic is not available. Guessing MIME types based on file extensions.
/tmp/tmpfile-NuTqRH8ZZd91i42z5GXE -> s3://dump/test/mytest  [1 of 1]
 40 of 40   100% in    0s    87.15 B/s  done
```
New output
```
cat mytest | s3cmd put -e - s3://dump/test/mytest
ERROR: Parameter problem: Cannot encrypt before uploading from stdin
```